Yichud between a brother and sister

Some Poskim[2] rule it is permitted for a brother and sister to remain alone together in the same room on a temporary basis.[3] [This applies even to a maternal or paternal sister.[4]] Other Poskim[5] rule it is forbidden for a brother and sister to remain together in Yichud even temporarily. Practically, the main ruling follows the lenient opinion and so is the custom of all Jewry.[6] Nevertheless, one who is stringent to not have Yichud with a sibling of opposite gender is blessed.[7] This especially applies in a promiscuous generation [and to children who are going through puberty during their pubescent and prepubescent years[8]].[9] [Some Poskim[10], however, rule that a sibling who is known to be promiscuous may not share even temporary Yichud with a sibling if opposite gender.] According to all opinions, it is Rabbinically[11] forbidden to have Yichud with a sibling of opposite gender on a permanent basis.[12] The Poskim debate the exact definition of permanent in this regard, as explained next.

Summary:

Temporary Yichud is permitted between siblings. It is thus permitted for them to live alone on a temporary basis. It is, however, forbidden for a brother and sister to live alone on a permanent basis. Those who are stringent to avoid even temporary Yichud with siblings are blessed.

Parents are to take heed to try to avoid situations in which their children of opposite genders remain alone together for extended periods of time. This especially applies towards children of pubescent, or prepubescent age. Children should be instructed that they are never allowed to lock the door of their room when they are there with a sibling of the opposite gender.

[5] Beis Shmuel E.H. 20:14 “And so too Yichud is forbidden” [unlike 22:1]; Radbaz 7:32; Yosef Ometz 22; Od Yosef Chaiy Shoftim 3 “Although there are Poskim who permit Yichud with a sister, the main opinion follows that it is forbidden, especially in this promiscuous generation.”; Shmuel in Kiddushin 81b [regarding all Arayos, including mother]; Implication of Rambam, Tur and Michaber who all omit the allowance of Yichud with a sister even though they mention the allowance of a mother [so leans to understand Beis Shmuel ibid; See Eiyn Yitzchak E.H. 8 and Nitei Gavriel 3 footnote 1 for an analysis on their opinion]; Implication of Kitzur SHU”A 152:1 who omits a sister

[6] The vast majority of Poskim [brought in footnote above] all rule it is permitted and that so is the main opinion, and so is the custom; Nitei Gavriel 3:1

The reason: The inclination for a mother or daughter is completely dormant and Yichud is hence allowed even permanently. However, the Yetzer for a sister although dormant is still possible, and hence the Sages limited the Yichud allowance to only a temporary basis. [See Rashi Kiddushin ibid; Pnei Yehoshua ibid; Tosafus Rosh Kiddushin ibid] Alternatively, it is not possible to decree against living alone with a mother or daughter, as opposed to a sister. [Erech Shaiy ibid]

[13] Heard from a child psychologist with decades of experience in dealing with incense and molestation cases that occur within the family